Conventionally, as the power unit of this kind, a power unit disclosed in Patent Document 1, for example, is known. This power unit is for driving left and right drive wheels of a vehicle, and is equipped with an internal combustion engine, which is a motive power source, and a transmission connected to the internal combustion engine and the drive wheels. The transmission includes first and second planetary gear units of a general single pinion type and first and second rotating machines each having a rotor and a stator.
As shown in FIG. 157, the first planetary gear unit has a first ring gear, a first carrier, and a first sun gear which are mechanically connected to the internal combustion engine, a second carrier of the second planetary gear unit, and the first rotating machine, respectively. The second planetary gear unit has a second sun gear, a second carrier, and a second ring gear which are mechanically connected to the second rotating machine, the drive wheels, and the first rotating machine, respectively. Moreover, the first and second rotating machines are electrically connected to each other through a controller. It should be noted that in FIG. 157, mechanical connections between elements are indicated by solid lines, and electrical connections therebetween are indicated by one-dot chain lines. Moreover, flows of motive power and electric power are indicated by thick lines with arrows.
In the conventional power unit configured as above, during traveling of the vehicle, the motive power from the internal combustion engine is transmitted to the drive wheels, for example, in the following manner. That is, as shown in FIG. 157, the motive power from the internal combustion engine is transmitted to the first ring gear, and is then combined with motive power transmitted to the first sun gear, as described later. This combined motive power is transmitted to the second carrier through the first carrier. Moreover, in this case, electric power is generated by the second rotating machine, and the generated electric power is supplied to the first rotating machine through the controller. In accordance with the electric power generation, part of the combined motive power transmitted to the second carrier is distributed to the second sun gear and the second ring gear, and the remainder of the combined motive power is transmitted to the drive wheels. The motive power distributed to the second sun gear is transmitted to the second rotating machine, and the motive power distributed to the second ring gear is transmitted to the first sun gear through the first rotating machine. Furthermore, the motive power of the first rotating machine generated along with the above-described supply of the electric power is transmitted to the first sun gear.